Loss of mental abilities represent a recognized threat to the quality of life of postmenopausal women with advancing age. Most recently, several studies found that younger women with breast cancer after chemotherapy and hormonal therapy had reduced mental abilities, such as the ability to remember or concentrate, compared to women who did not undergo these treatments. These studies indicate that the agents used for breast cancer treatment and prevention cause brain damage. In the previous studies, most of the women with mental deficits obtained both chemotherapy and hormonal therapy, so that it is unclear which of the two therapies caused the mental deficits. The goal of this study is to determine if women with breast cancer receiving tamoxifen, an "anti-estrogen," as treatment have more mental deficits and abnormal neurochemistry compared to healthy women.